1. Technical Field
The invention relates generally to semiconductor packaging, and more particularly, to a semiconductor package including non-aligned active vias in adjacent layers.
2. Background Art
Advances in flip chip plastic ball grid array (FCPBGA) laminate technology continue to push the state of the art for electrical performance and reliability. For new applications containing high speed signal (HSS) lines, it is critical for signal integrity to have the signal lines isolated from power and ground planes. This is easily designed into HSS lines routing the signal through the FCPBGA laminate structure. However, to provide electrical connections to a card or board, these lines must end in a ball grid array (BGA) joint. This situation often requires routing the line down through power or ground planes.
When a HSS line is routed through a power or ground plane, there needs to be a clearance, or a gap, between the signal and power or ground to protect signal integrity. In the case of the connection to a BGA, this clearance feature means that the microvia connecting the BGA pad to the next layer needs to provide the mechanical robustness to ensure that the BGA balls do not sever during normal handling for assembly and test. Where the BGA pad is connected to the second or third layer away, this can be accomplished by a stacked via structure, which increases the risk of an electrical open due to mechanical damage.
The use of multiple vias to connect a BGA pad to a power or ground plane is common and known in the art. Typical connections utilize 2-5 microvias per BGA pad to connect to the power or ground plane, which is typically the next layer up from the BGA pad. However, the purpose of having multiple vias is to reduce the electrical current flowing through a single via by providing multiple parallel paths for the current. The mechanical robustness of a BGA ball connected to a power or ground plane has not been a concern.
HSS connections are more susceptible to mechanical damage than power and ground connections. It is therefore desirable to provide more mechanical robustness to HSS BGA via connections. One approach to this problem is to increase the via land diameter or provide metal layers as reinforcement, thus making the HSS BGA via stacks more closely resemble the power and ground structures. While this approach has advantages, there are limitations due to the electrical isolation requirements from the surrounding power and ground features.